Recently, from the viewpoint of raise in environmental consciousness, liquid fuels in which the contents of environmental load substances such as sulfur and aromatic hydrocarbons are small have been demanded. From such a viewpoint, as a technique which can produce a base stock for fuel oil that substantially contains neither sulfur nor aromatic hydrocarbons and is rich in aliphatic hydrocarbons, particularly, base stock for kerosene and gas oil, a technique that has been attracting attention is one which synthesis gas (mixed gas containing carbon monoxide gas and hydrogen gas as main components) is produced from a hydrocarbon source such as natural gas by a reforming reaction, hydrocarbons are synthesized from this synthesis gas by a Fischer-Tropsch synthesis reaction (hereinafter, also referred to as the “FT synthesis reaction”), and the hydrocarbons are further refined by hydroprocessing and fractionation to thereby obtain a base stock for fuel oil (see Patent Literature 1, for example). This technique is called GTL (Gas To Liquids) process.
A synthetic oil obtained from synthesis gas by the FT synthesis reaction (hereinafter, also referred to as the “FT synthetic oil”) is a mixture containing aliphatic hydrocarbons with a wide carbon number distribution as a main component. A naphtha fraction, a middle distillate fraction, and a wax fraction can be obtained by fractionating this FT synthetic oil according to boiling points. Among the respective fractions, the middle distillate is the most useful fraction corresponding to base stock for kerosene and gas oil and is desired to be obtained with a high yield.
Hydrocarbons can be obtained with a boiling point range comparable to the middle distillate by hydrocracking a wax fraction contained in a significant amount in combination with the middle distillate in the FT synthetic oil, and the useful middle distillate can be obtained with a high yield from the FT synthetic oil by using this method.
In the hydrocracking of a hydrocarbon raw oil containing the above-described wax fraction, it is necessary to enhance the cracking rate of the wax fraction, for enhancing the yield of the middle distillate; on the other hand, there is a problem that if the cracking rate is enhanced, the production of the light fraction attributed to excessive hydrocracking raises and rather the yield of the middle distillate reduces. Thus, as a hydrocracking catalyst used in the hydrocracking of a hydrocarbon raw oil containing the wax fraction, it is required to have a high hydrocracking activity and also to suppress the production of the light fraction attributed to excessive hydrocracking and give the middle distillate with a high selectivity. As such a hydrocracking catalyst, a catalyst comprising a catalyst support comprising a zeolite and an amorphous composite metal oxide having solid acidity and an active metal supported by the catalyst support and selected from noble metals of Group 8 to Group 10 in the periodic table is known (see Patent Literatures 2 and 3, for example).